This invention generally relates to methods for collecting airborne particulates, and more specifically, to methods for collecting and archiving airborne particulates using an impact collector.
The separation and collection of particulates/aerosols from an airstream (or other fluid streams) is of concern in several contexts. In some cases, the goal may be to simply remove the particulates/aerosols from the fluid stream, thereby cleaning or purifying the fluid. Often it is desired to remove all particulates, regardless of composition, if the particulates are above a certain size. For example, automobile painting and the fabrication of silicon chips in clean rooms represent two situations in which all particulates large enough to result in an inferior product are desirably removed from the processing environment.
In other cases, particulates are collected for analysis to determine the type and concentration of such particulates/aerosols entrained in the fluid. For example, this technology may be employed in the detection of airborne biological or chemical warfare agents, the detection of biological contamination in confined spaces, such as aircraft or hospitals, or the detection of industrial pollutants (either in ambient fluid or in the effluent of smokestacks).
Much effort has been expended in the past in the detection and classification of particulates or aerosols in fluid streams. Impactors have been used for collecting aerosol particulates for many decades. In the earliest embodiments, a stream of fluid containing the particulates was accelerated toward an impactor plate. Due to their inertia, the particulates striking the impactor plate were collected on its surface, while the fluid was deflected to the side. With these types of impactors, only larger particulates could be collected, since particulates below a certain xe2x80x9ccut sizexe2x80x9d were carried away by the fluid stream.
However, a significant disadvantage of such an impactor is the deposition of particulates on surfaces of the impactor other than the intended collection surfaces. This phenomenon reduces the accuracy of measurement of total particulate mass concentration and of the size-fractionation of particulates, since such losses cannot be accurately estimated for aerosols or particulates of varying size, shape, or chemistry. Additionally, particulates may either become re-entrained in the fluid stream, or may bounce off the impactor""s collection surface upon impact. To remedy this problem, xe2x80x9cvirtualxe2x80x9d impactors have been developed that separate particulates from a fluid stream with techniques other than direct impaction. Virtual impactors may operate on a number of different principles, but all avoid actual xe2x80x9cimpactxe2x80x9d as a means to separate particulates from a fluid in which the particulates are entrained and rely on differences in particulate mass to induce inertial separation. Specifically, a particulate-laden fluid stream is directed toward a surface presenting an obstruction to the forward movement of the fluid stream. The surface includes a void at the point where the particulates would normally impact the surface. When a major portion of the fluid stream changes direction to avoid the obstruction presented by the surface, fine particulates remain entrained in the deflected major portion of the fluid stream. Heavier or more dense particulates, on the other hand, fail to change direction and are collected in a region of relatively stagnant fluid (a xe2x80x9cdead zonexe2x80x9d) that is created near the surface. The heavier particulates entrained in a minor portion of the fluid stream enter the void defined through the surface, where they can be captured or analyzed.
Some examples of virtual impactors can be found in U.S. Pat. Nos. 3,901,798; 4,670,135; 4,767,524; 5,425,802; and 5,533,406. Because typical virtual impactors do not actually collect particulates themselves, but merely redirect them into two different fluid streams according to their mass, they are essentially free of the problems of particulate bounce and particulate re-entrainment associated with actual impactor devices. Still, particulate xe2x80x9cwall loss,xe2x80x9d i.e., unintended deposition of particulates on various surfaces of virtual impactor structures, especially at curved or bent portions, remains a challenge with some designs of virtual impactors, because typically, many stages or layers of virtual impactors are required to complete particulate separation.
An additional aspect of the collection of fluid-entrained particulates, especially with respect to particulates that will be analyzed to determine a type and concentration of particulates, relates to when the collected particulates are to be analyzed. A common practice is to sample a fluid for a period of time, and then analyze the collected sample immediately, or at least as soon as practical. Depending on the nature of the particulates for which the fluid is being sampled, immediate analysis may be required. For example, if chemical or biological agents that pose an immediate health threat are suspected, real time analysis is preferred to enable protective measures to be taken immediately, before irreversible harm can occur. However, there are also many applications, such as routine monitoring of smokestacks and waste water discharge, in which only a portion of the collected sample might need to be analyzed shortly after collection, while other portions are best archived for later analysis.
Archival samples can be prepared by taking a collected sample and manually splitting that sample into various fractions, including a first fraction to be analyzed relatively soon, and one or more additional portions to be archived for possible later analysis. While archival samples prepared by such a method are useful, the manual nature of such a method is time consuming and costly. Furthermore, during each step in which a sample is handled or manipulated (collection, separation, storage, and analysis), there is a significant chance that the sample will be inadvertently contaminated. It would thus be desirable to provide a method and apparatus that more readily enables archival samples to be prepared, with a minimal risk of contamination.
It should also be noted that the manner in which samples are collected affects the usefulness of the samples for archival purposes. Archival samples are often employed to determine more information about an event occurring at a specific time. For example, archival data collected from a smokestack might be used to determine at what time higher emissions occurred. That time could then be applied to analyze the process and equipment utilizing the smokestack to isolate the factors causing the excess emissions, so that the problem can be corrected. If the archival sample is merely a single sample collected over a 24-hour period, rather than 24 samples collected each hour for 24 hours, then little information can be obtained about when the excess emissions actually occurred, making it more difficult to determine the cause of the excess emissions. It would be therefore be desirable to provide a method and apparatus capable of providing archival samples for successive relatively short sampling periods, and which include time indexing enabling a specific archival sample to be correlated with a specific time at which the sample was taken.
Accordingly, a need exists to develop a method and apparatus capable of providing time-indexed archival samples with minimal operator effort, and minimal chance of contamination. Such archival samples desirably should include a high concentration of particulates, so that the archival samples are compact and require minimal storage space. Preferably, a virtual impactor that efficiently separates particulates from a fluid stream might be employed to collect the particulates.
The present invention is directed to a method and apparatus for concentrating, collecting, and depositing xe2x80x9cspotsxe2x80x9d of particulates from a fluid onto a solid, archival quality medium. Such an archive, in the form of many spots collected at regular time intervals from a specific site, will enable investigation of environmental conditions (based on collected particulates) at a future time. Archived samples, each relating to a specific period of collection, can be stored and later analyzed to quantitatively and/or qualitatively test for a specific particulate at a specific time. It is anticipated that such archives will be very useful in the study of potentially hazardous particulates, including but not limited to viruses, bacteria, bio-toxins, and pathogens. Those of ordinary skill in the art will readily recognize that such archived samples can be analyzed using a variety of known analytical techniques including, but not limited to, mass spectrophotometry.
The present invention works best if fluid-entrained particulates (most often airborne particulates) are efficiently collected and concentrated, a task for which a virtual impactor, such as described in parent application Ser. No. 09/191,980, is ideal. Also, it is important to provide both a suitable archival quality surface for collecting concentrated spots of particulates to be deposited on, as well as providing means for moving the archival surface relative to the concentrated stream of particulates over time, so that spots located on different portions of the archival surface correspond to different increments of time.
Preferably the invention includes means for associating a date and time with each spot for the purpose of accurate archiving, which can be achieved in many ways including, but not limited to, a computer program that records the date and time at the instant of spot deposition, saving the data to a file for later reference. Preferably, the archival surface employed can accommodate many spots in a limited area, collected at intervals over a long period of time.
The invention is also preferably able to accept a variety of sample protocols that determine when the fluid (most often air) is sampled to produce a spot. These sample protocols, e.g., programs executed on a computing device or a hard wired logic device, can be quite simple, at times comprising only a timer that determines the waiting period between samples. Alternatively, the sample protocols can be more complex, such as protocols that comprise a schedule for sampling at variable intervals, which depend on environmental conditions determined using sensors.
The archival surface onto which the concentrated particulates are directed can be selected or modified to enhance a deposition of the particulates onto the archival surface. In some embodiments, the material of the archival surface has been selected because of its porous nature. The pore sizes are selected to be large enough to allow the fluid the particulates are entrained in to freely pass through the archival surface, and small enough to prevent the particulates themselves from passing through the archival surface. Thus the particulates are xe2x80x9cfilteredxe2x80x9d out of the fluid stream by the archival surface. In other embodiments, the archival surface is coated with a material selected to enhance a deposition of the particulates onto the archival surface. Such surfaces generally promote adhesion via chemical attraction, (i.e. a hydrophobic-hydrophobic attraction, or a hydrophilic-hydrophilic attraction). Electrical attraction can also be employed (i.e. a positively charged surface for collecting negative particles).
In at least one embodiment, the virtual impactor includes a separation plate employed for separating a fluid stream into a major flow and a minor flow. The major flow includes a minor portion of particles that are above a predetermined size, and the minor flow includes a major portion of the particles that are above the predetermined size. The separation plate includes a block in which is defined a laterally extending passage having an inlet disposed on one edge of the block and an outlet disposed on an opposite edge of the block. This laterally extending passage has a lateral dimension that is substantially greater than a transverse dimension of the passage. Opposed surfaces of the passage between which the transverse dimension of the passage is defined generally converge toward each other within the block, so that the outlet has a substantially smaller cross-sectional area than the inlet. A transverse, laterally extending slot is defined within the block and is in fluid communication with a portion of the passage that has the substantially smaller cross-sectional area. A major flow outlet port is also defined in the block, in fluid communication with the transverse, laterally extending slot. The major flow enters the slot and exits the block through the major flow outlet port, while the minor flow exits the block through the outlet of the passage. The major flow carries the minor portion of the particles and the minor flow carries the major portion of the particles.